The present invention primarily relates to a method for manufacturing positive and negative electrodes of a nonaqueous-electrolyte rechargeable battery.
In recent years, there has been an increasing trend of electronic equipment towards portable and cordless design, and demand for small and lightweight rechargeable batteries having high energy density has been growing. Among others, nonaqueous-electrolyte rechargeable batteries typified by lithium rechargeable batteries using lithium as an active material are particularly hoped for as batteries having a high voltage and a high energy density. However, a positive electrode plate, a negative electrode plate, and a separator of such nonaqueous-electrolyte rechargeable batteries are respectively formed independently. A separator that generally occupies a largest area, a negative electrode plate, and a positive electrode plate, in that order, must be smaller in area. Consequently, there is an area that is occupied only by a separator. As a result, a portion of a separator that is not involved in a battery capacity largely occupies a space in the battery, reducing discharge capacity per unit volume of battery.
Further, because a separator, a positive electrode plate, and a negative electrode plate respectively independently exist, a clearance is produced between the separator, and the positive electrode plate or the negative electrode plate. Gases produced by reactions between electrode plate surfaces and the electrolyte, etc. during charging and discharging accumulate in the clearance, deteriorating battery characteristics.
Methods for unitizing an electrode plate and a separator include steps for polymer batteries using heat-fusing (U.S. Pat. No. 5,460,904). With these steps, a separator and an electrode plate are independently prepared and are unitized together by heat-fusing. Further, to make the separator a porous membrane, the steps include a step in which the separator is made to contain a plasticizer when forming a separator membrane, and the plasticizer is extracted after the heat-fusing. Accordingly, the steps are very complex, inviting disadvantages such as lowering in productivity and rise in costs.
Further, alternative methods for unitizing an electrode plate and a separator include a step of coating an insulation layer on an electrode plate, and are disclosed in Japanese Patent Laid-Open Publication Nos. Hei. 10-50348, Hei. 11-288741, and others. With a method disclosed in Japanese Patent Laid-Open Publication No. Hei. 10-50348, a polyethylene wax is heated and melted, and is coated on an electrode plate, and after that the coated layer is perforated with means such as a thermal head. It means that, with this method, the coated resin layer becomes a homogenized membrane in any case, and the layer must be perforated by some means or other to make it a porous membrane so that the layer functions as a separator. Further, in Japanese Patent Laid-Open Publication No. Hei. 11-288741, a typical polymer film forming method is disclosed, in which a polymer material is melted in a solvent, and the solution is coated on an electrode plate. However, because a solvent having an excellent dissolving ability for a polyolefin resin such as polyethylene is not available, preparing an insulation layer membrane with such a common method is extremely difficult. Therefore, in this Patent Publication as well, polyolefin resins are not referred to as a polymer material.
The present invention has been made in light of the conventional problems described above, and an object of the invention is to provide a method for manufacturing a battery electrode plate in which the electrode plate uses as a separator a polyolefin resin such as polyethylene, which is a material relatively low in costs and is stable for use in a battery, a positive electrode plate or a negative electrode plate is unitized with an insulation layer through the coating of the polyolefin resin on the positive electrode plate or the negative electrode plate. Thus the volume of a separator in the space within a battery is minimized, and interfacial bonding between the positive electrode plate or the negative electrode plate and the insulation layer is improved, thereby enhancing battery characteristics.
In order to achieve the object described above, a method for manufacturing a battery electrode plate according to a first aspect of the invention includes the steps of: mixing a solvent with a polyolefin resin used as an insulation layer of a positive electrode plate or negative electrode plate; preparing a gel-like solution, or a gelled solution as a whole having a high viscosity, by heating the mixture of the polyolefin resin and the solvent at a temperature at which a part or the whole of the polyolefin melts; forming an insulation layer by coating the gel-like solution on a surface of the positive electrode plate or negative electrode plate; and drying it by heating the positive electrode plate or negative electrode plate formed with the insulation layer.
In this method for manufacturing a battery electrode plate, the insulation layer that corresponds to a separator and the positive electrode plate or negative electrode plate are made equal in area. Thereby, a portion conventionally occupied only by a separator is utilized, so that discharge capacity per unit volume of battery is improved. At the same time, since the insulation layer and the positive electrode plate or the negative electrode plate are unitized together, better interfacial bonding is provided in comparison with a conventional separator, so that battery characteristics is improved.
Further, a method for manufacturing battery electrode plate according to a second aspect of the invention includes the steps of: mixing a polyolefin resin with a solvent; preparing a gel-like solution, or a gelled solution as a whole having high viscosity, by heating the mixture at a temperature where a part or the whole of the polyolefin melts; adding a fluororesin and/or an imide resin to the polyolefin resin at any stage between the state where the polyolefin resin exists alone and the state of the gel-like solution; coating the gel-like solution on a surface of a positive electrode plate or negative electrode plate; and drying it by heating the positive electrode plate or the negative electrode plate coated with the gel-like solution to form the gel-like solution into an insulation layer on the positive electrode plate or negative electrode plate.
According to the method for manufacturing an electrode plate according to the second aspect of the invention, heat resistance of the insulation layer formed mainly from a polyolefin resin is improved. Among different resins, a polyolefin resin has a lower melting temperature. When a battery using a polyolefin resin is exposed to an environment of high temperatures exceeding a melting temperature of a polyolefin resin due to users"" mishandling, the polyolefin resin may melt to deform and drop. However, the added fluororesin and/or imide resin having a higher melting temperature provides a state in which particles of resins are connected together, and fluororesin and/or imide resin that is not melted prevents deformation and drop of the polyolefin resin.